The present invention relates to a method as defined in the preamble of claim 1.
German Auslegeschrift [Published Patent Application] No. 2,033,074 discloses a method for melting fission products into a glass mass, in which a quantity of the molten glass is periodically extracted from the bottom of a ceramic glass melting furnace by temporarily melting a plug of the glass mass which seals a discharge nozzle provided at the bottom of the glass melting furnace by means of induction heating. This discharge nozzle includes a metallic pipe and is inserted into the bottom of the ceramic glass melting furnace. It may be heated by means of an induction heater which is also inserted into the bottom of the furnace. Although the termination of the discharge of a glass melt charge is now described in this publication, this is probably effected by switching off the heater at the outlet and, as a result, slowly cooling the ceramic material surrounding the outlet.
This method has the following drawbacks:
1. During melting of the glass plug in the outlet, not only the discharge nozzle but also the ceramic material surrounding it is heated to almost the melting temperature of the glass plug;
2. Termination of the extraction of a charge requires a relatively long period of time after the heater is switched off until a sealing plug is again formed in the discharge nozzle by cooling of the subsequently discharged glass melt; and
3. Formation of a glass tongue, or a glass thread, at the end of the discharge nozzle cannot be avoided.
German Offenlegungsschrift [Laid-open Application] No. 2,426,328 discloses a tank furnace for continuously melting glasses, ceramic frits, minerals, glazings and enamels in which the molten glass is intended to be discharged in a simple manner. The outlet from this tank is oriented vertically downwardly and is disposed at the lowest point of the tank. A tubular electrode is disposed in the opening of the ceramic outlet block and the glass flows through a bore in the electrode. The electrode is placed onto an annular channel through which cooling water flows and which serves the purpose of cooling the entire lower face of the outlet block. This publication mentions that the outlet block constitutes the weakest part of the furnace since the entire discharge is effected through a relatively small bore. It is therefore necessary, the publication states, to protect the outlet against excessive corrosion by producing extensive cooling. Overly intensive cooling of the melt or the ingress of undesirable air into the exiting melt is prevented by a ceramic pipe which is connected at the bottom of the cooling channel. The melt itself is heated by further electrodes inside the tank. Intermittent discharging of the melt is not mentioned in this publication and is probably not intended, because it is pointed out that with this tank the desired discharge can be kept constant so that too rapid an emptying or too slow a discharge can be avoided. It is asserted as an advantage that the available melt can be discharged at a controlled rate.